I sympathize with the clay soil, having started my gardening in Ohio clay. I would amend the whole bed rather than just the planting hole. In clay soil, if you only amend the planting hole, you often find the the roots rot because the clay soil forms a bathtub effect where the water flows into the lose soil in the planting hole during rain or irrigation and then doesn’t drain out due to the tight clay soil around the planting hole. Instead, amend the whole bed, or add amendments to the surface so you don’t have the change in soil texture. Something with a coarser texture than peat should work better. Considering the time of year, consider chopped leaves. (Run the mower over a pile of leaves a few times.) Or you can plant a bit high so that the plant is half in a hole and half in mounded up soil so you don’t get the bathtub effect.

Your Hydrangea (which sounds like a big leaf Hydrangea/Hydrangea macrophylla) should be fine in all day dappled sun or if you have a spot with morning only sun. Be sure to water it well and keep an eye on moisture level since tree roots can be thirsty.

I have some oakleaf, serratas and mopheads in dappled sun and bloom. Dense shade is what can reduce bloomage markedly.

I plan to amend the entire bed. I have a huge pile of mulched leaves but they haven't turned into "gold" yet. I have a composter for kitchen waste but it's not quite ready, either. What amendment would be better than peat? I know they prefer loamier soil but I don't want to add sand.

Get a bag of organic compost if nothing else is handy.

If it is a smaller bed, go ahead and add the partly composted leaves and partly composted kitchen waste and turn it in well. Once it is turned in, it will finish decomposing. Add a top layer of mulch to keep it looking tidy and to keep moisture even. Or for a large whole bed, get a truckload of compost from a soil supplier or if your municipality sells or gives away compost, get a bunch of that.